Do not cache the result of RenderStyle::computedLineHeight()

We currently use 26-bits in RenderBlock to cache the result of
RenderStyle::computedLineHeight(). However when you look at the
computations behind that function the main components are already cached
so at most we are saving a handful of computations per call.

In https://codereview.chromium.org/256593009/ we added a test that calls
RenderBlock::lineHeight() ~425,000 times on each run. This CL results in
no measurable degradation in performance against that test.

So since we cannot get a measurable improvement in performance as a result
of the caching when testing locally, remove it and free up space for more useful
bits in RenderBlock. We will let it cycle through the perf bots for a while
before allowing people to start using the spare bits though.

The cached result originally sat in RenderText and was inherited from the
fork from KHTML. http://trac.webkit.org/changeset/6010 moved it to RenderBlock
on the basis that "calls to fontMetrics().lineSpacing() are expensive." This
is no longer the case as far as I can tell, since both lineSpacing() and
fontMetrics() are now cached themselves.

Review URL: https://codereview.chromium.org/260073005

git-svn-id: svn://svn.chromium.org/blink/trunk@176233 bbb929c8-8fbe-4397-9dbb-9b2b20218538